Who knew? Brains with Alzheimer’s can work just fine.

Almost all of the dozen or so people I’ve interviewed so far feel that keeping active forestalls decline. Billy Kyle is typical. When his son joined his Detroit general surgery practice, he left the hospital and now performs only outpatient surgeries in the office. “But I didn’t want to stop, because I think when you stop you start going down,” he comments. “I think when you get to the point that you want to retire, you have to retire to something else. I get up at 6, 6:30 in the morning and I think I need to keep doing that: get out of the bed, get cleaned up, and do something.”

As a physician, Kyle observes the atrophy that sets in when people aren’t physically active — it’s obvious. But what about mental activity? Are all the older Americans shelling out for computer games designed to exercise their brains being hoodwinked? Will Alzheimer’s claim some random percentage of us, no matter how much Sudoku we cram in before bedtime?

It seems not. Jane Brody’s 12/11/07 Health column in the New York Times points out that “many people remain sharp as a tack well into their 80s and beyond.” I figured that that was the luck of the draw: the dementia needle hadn’t come to a halt over their little slice of the game board. Here’s the kicker: “But when these sharp old folks die, autopsy studies often reveal extensive brain abnormalities like those in patients with Alzheimer’s.”

Why don’t their mental states reflect this pathology? Because of what the Columbia-Presbyterian Medical Center doctors who conducted the study term “’cognitive reserve.’ Cognitive reserve, in this theory, refers to the brain’s ability to develop and maintain extra neurons and connections between them via axons and dendrites. Later in life, these connections may help compensate for the rise in dementia-related brain pathology that accompanies normal aging.” It’s strange to think that you could die with a brain riddled with plaques and never know it, but it’s happy-making. And what builds cognitive reserve is ongoing physical and mental stimulation.

Luck matters: genetic intelligence comes into play, and cognitive reserve correlates with higher levels of education. But while it’s best to start early, “there is evidence that this account can be replenished even late in life.” The key? Resist habit and inertia. Like muscles, “the brain requires continued stresses to maintain or enhance its strength.” Challenge yourself with harder puzzles, different workouts, meeting strangers on alien turf. No Barcaloungers and re-runs, baby.